Investigating the dorsal and ventral pathways of reading through behavioural and neural markers: a multimodal approach using tdcs and eeg

Reading is a skill that requires processing of orthography, phonology, and semantics associated with a word. Contemporary brain network models help us understand how reading occurs through a combination of a dual route and a connectionist model. The dual route model of reading, proposes reading to involve by two independent parallel neuronal pathways in the left hemisphere: (1) dorsal route involved in sub-lexical grapheme-to-phoneme conversion comprising of inferior parietal lobule, superior temporal gyrus, and inferior frontal gyrus (2) ventral route comprising the semantic access from orthography encompassing left middle temporal gyrus, basal temporal area, and inferior frontal gyrus. This model provides explanation for frequent and regular words to be processed faster. Building on this, the connectionist dual process model (CDP) proposes the existence of semantic connections between the two pathways (dorsal and ventral) in parietal and temporal lobes to further explain the effect of subtle lexical variations on word recognition process. The CDP model is supported by neuroimaging studies, however the resultant conclusions are correlational in nature. Thus definitive inferences about the involvement of a particular brain region underlying a behaviour is difficult. In this aspect, non-invasive brain stimulation technique such as transcranial Direct Current Stimulation (tDCS) which directly modulate the targeted brain region could provide more causal evidence. tDCS passes a very low amount of current through the cortex via two or more electrodes positioned over the skull (referred to as tDCS montage). Previous studies have found that reading performance in both typical and atypical readers is enhanced following tDCS. However, meta-analyses across these studies reported inconsistency in the findings. This could be attributed to three reasons (1) variability in the tDCS montages , 2) variability in reading task , 3) lack of functional segregation of dorsal and ventral pathways , across the studies. Furthermore, attempts to establish a neural marker underlying tDCS induced changes in reading behaviour has not been made in previous studies. Given these research gaps, the present thesis aims to achieve three research objectives (1) determining the target specific montage for dorsal and ventral pathways through computational modelling, (2) apply these montages on reading tasks that involves either lexical or sub-lexical processing, and (3) specifying the neural correlates (marker) of reading from electroencephalogram (EEG) recordings with and without tDCS. Friday 24 Apr 2020